1. Field of the Invention
The present invention relates to integrated resistors. It more specifically relates to resistors made of polysilicon in an integrated circuit.
FIG. 1 shows in a very simplified partial perspective view, an example of a polysilicon resistor to which the present invention applies.
2. Discussion of the Related Art
Such a resistor 1 is formed of a polysilicon track (also called a bar) obtained by etching a layer deposited on an insulating substrate 2. Substrate 2 is directly formed of the integrated circuit substrate or is an insulating layer forming integrated circuit substrate or the like for resistor 1. Resistor 1 is connected, by its two ends, to conductive tracks (for example, metal tracks) 3 and 4 intended to connect the resistive bar to the other integrated circuit elements according to the application. The simplified representation of FIG. 1 makes no reference to the different insulating and conductive layers generally forming the integrated circuit. To simplify, only resistive bar 1 laid on insulating substrate 2 and in contact, by the ends of its upper surface, with the two metal tracks 3 and 4, has been shown. In practice, the connections of resistive element 1 to the other integrated circuit components are obtained by wider polysilicon tracks starting from the ends of bar 1, in the alignment thereof. In other words, resistive element 1 is generally formed by making a section of a polysilicon track narrower than the rest of the track.
Resistance R of element 1 is given by the following formula:R=ρ(L/s),where ρ designates the resistivity of the material (polysilicon, possibly doped) forming the track in which element 1 is etched, where L designates the length of element 1, and where s designates its section, that is, its width l multiplied by its thickness e. Resistivity ρ of element 1 depends, among others, on the possible doping of the polysilicon forming it. In certain cases, the polysilicon element is covered with a metal layer, the resistive element then combining the polysilicon and the overlying metal.
Most often, upon forming of an integrated circuit, the resistors are provided by referring  to resistance per square R□. This square resistance is defined as being the resistivity of the material divided by the thickness with which it is deposited. Taking the above relation giving the resistance of an element 1, the resistance is thus given by the following relation:R=R□*L/l.
Quotient L/l corresponds to what is called the number of squares forming resistive element 1. This represents, as seen from above, the number of squares of given dimension depending on the technology, put side by side to form element 1.
The value of the polysilicon resistance is thus defined, upon manufacturing, based on the above parameters, resulting in so-called nominal resistivities and resistances. Generally, thickness e of the polysilicon is set by other manufacturing parameters of the integrated circuit. For example, this thickness is set by the thickness desired for the gates of the integrated circuit MOS transistors.
In recent technologies, the use of polysilicon resistors is limited to resistors that carry, in operation, currents smaller than 100 μA. For greater currents, a diffusion resistor is generally used. Polysilicon is however preferred to a dopant diffusion, since the occurrence of stray capacitances with the substrate is avoided.
It would be desirable to be able to modify the value of the resistance of a polysilicon element (1, FIG. 1) after manufacturing. Up to now, the only envisaged modification is a physical deterioration (melting), a strongly conductive polysilicon element being used as a fuse. For this purpose, a very high current is imposed (on the order of one tenth of an ampere) in the polysilicon element to cause a physical deterioration thereof and obtain an open circuit (fuse).